Add a manual workflow that publishes a Gitea release to Modrinth

"Publish to Modrinth" (workflow_dispatch, tag + dry_run inputs) takes the
jars the Release workflow attached to a Gitea release and creates one
Modrinth version per jar: version_number <version>+<mc>, that Minecraft
version, fabric loader, Fabric API as a required dependency, the release
notes as the changelog, and alpha/beta/release from the tag. It doesn't
rebuild, skips versions already on Modrinth, and downloads jars through
the public URL (Gitea's reported URLs use its internal ROOT_URL).

Needs a MODRINTH_TOKEN secret with the "Create versions" scope.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

.gitea/workflows/modrinth.yml

@@ -0,0 +1,40 @@
+name: Publish to Modrinth
+
+# Publishes the jars already attached to a Gitea release (built by the Release
+# workflow) to Modrinth, one version per Minecraft version. Run it manually
+# after smoke-testing the release. It doesn't rebuild anything, and versions
+# already on Modrinth are skipped, so re-running is safe.
+#
+# Needs a MODRINTH_TOKEN repository secret: a Modrinth PAT with the
+# "Create versions" scope.
+on:
+  workflow_dispatch:
+    inputs:
+      tag:
+        description: Tag of the Gitea release to publish
+        required: true
+      dry_run:
+        description: Only check and print what would be published
+        type: boolean
+        default: false
+
+jobs:
+  publish:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - name: Set up Python
+        uses: actions/setup-python@v5
+        with:
+          python-version: "3.12"
+
+      - name: Publish to Modrinth
+        env:
+          TAG: ${{ inputs.tag }}
+          DRY_RUN: ${{ inputs.dry_run }}
+          SERVER_URL: ${{ github.server_url }}
+          REPO: ${{ github.repository }}
+          MODRINTH_TOKEN: ${{ secrets.MODRINTH_TOKEN }}
+        run: python .gitea/scripts/publish_modrinth.py